Machines relating to injection molding machines are generally known. To produce a part from thermoplastic material, granulated plastic is plasticized in an injection molding machine and is metered into the space in front of the screw, and a back pressure is built up in the melt. Usually, the melt is injected into a tool cavity by moving the screw axially. The melt pressure is maintained, that is a holding pressure is built up, so as to compensate the natural material shrinkage. For example, EP 0 662 382 A1 describes an injection unit which is operated by electric motors and which likewise operates by the injection molding process described above. However, here the back pressure in the melt is created by an additional hydraulic apparatus.
A disadvantage of the known methods used by these electric injection molding machines is that an independent hydraulic system, among other things, is used to create and control the back pressure. Because of its complexity, such a system can be very costly. The use of an electric direct drive, where the nut is an integral component of the motor, is a source of disadvantages for this system, as regards cooling, maintenance (lubrication), and service. The motor (metering motor) is held fixed in its position (rpm 0) during injection and during the holding pressure. This results in a relatively high current load on the electronic power sections as long as the injection process lasts. This can raise the temperature above permissible limits, unless the current is reduced early on. This circumstance greatly reduces the capability of this machine in elastomer applications, where extremely long injection and holding pressure times are required.
In accordance with one aspect of the invention, a machine is provided for operating an injection unit through two electric motors, which is capable of all required injection processes, requires no additional hydraulic equipment, and which is designed for easy maintenance.
In accordance with a particular aspect, a space is formed by at least two sleeves, which can move relative to one another, and by respectively adjoining plate members and hubs so as to take up lubricating oil, such that the sleeves can slide into one another, and the junctions of the parts which form the space are constructed tight against the environment, at least for lubricants. When the screw moves axially, the sleeves push themselves into one another and reduce or enlarge the space. Since this space is filled with lubricant, the relative axial motion of the sleeves, that is a kind of pumping motion, transports the lubricant to the bearing points.
The two motors can be seated offset next to one another, so that they can act on the axle of the screw without any problem. No deflections are thus required.
Another development specifies that a coupling is situated between the screw and the spindle unit, so as to prevent the screw from rotating during the axial motion of the spindle. The coupling can be driven electrically but also hydraulically.